The present invention relates to a liquid crystal display, more particularly to a liquid crystal display having an improved picture quality.
The In-Plane Switching (hereinafter IPS) mode liquid crystal display has been suggested to complement the viewing angle characteristic of Twisted Nematic (hereinafter TN) mode liquid crystal display. (Principle and Characteristic of Electro-optical behavior with in-plane switching mode, Asia Display 1995, p. 577-580).
The IPS mode liquid crystal display is illustrated in FIG. 1. Electrodes 2a,2b for driving a liquid crystal molecule 3a are disposed on a lower substrate 1. At that time, a distance between the electrodes 2a,2b is larger than a distance between the lower substrate 1 and an upper substrate (not shown) thereby forming an electric field parallel to a surface of the lower substrate 1. Herein, the electrodes 2a,2b are made of opaque material, such as Mow and Al. A reference symbol xe2x80x9cRxe2x80x9d stands for a rubbing direction of an alignment layer.
As known in the art, the refractive anisotropy (or birefringence, xcex94n) is occurred due to the difference of the lengths of the long and the short axes. The refractive anisotropy xcex94n is also varied from the viewer""s viewing directions. Therefore a predetermined color is appeared on the region where the polar angle is of 0 degree and the azimuth angle range of degrees 0, 90, 180 and 270 in spite of the white state. This phenomenon is regarded as color shift and more detailed description thereof is attached with reference to the equation 1.
T≈T0 sin2 (2"khgr")xc2x7sin2 (xcfx80xc2x7xcex94nd/xcex)xe2x80x83xe2x80x83equation 1
Wherein,
T: transmittance;
T0: transmittance to the reference light;
"khgr": angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate;
xcex94n: birefringence;
d: distance or gap between the upper and lower substrates (thickness of the liquid crystal layer); and
xcex: wavelength of the incident light.
So as to obtain the maximum transmittance T, the "khgr" should be xcfx80/4 or the xcex94nd/xcex should be xcfx80/2 according to the equation 1. As the xcex94nd varies with the birefringence difference of the liquid crystal molecules from viewing directions, the value of xcex is varied in order to satisfy xcfx80/2. According to this condition, the color corresponding to the varied wavelength xcex appears.
Accordingly, as the value of xcex94n relatively decreases at viewing direction xe2x80x9cxcex1xe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases also. Consequently a color of blue having a shorter wavelength than that of a color of white appears.
On the other hand, as the value of xcex94n relatively increases at a viewing direction xe2x80x9cxcex2xe2x80x9d toward the short axes of the liquid crystal molecules, the wavelength of an incident light relatively increases also.
Consequently a color of yellow having a longer wavelength than that of the color of white appears.
Deterioration is caused in the picture quality of IPS mode liquid crystal display.
Furthermore, since the electrodes 2a,2b are made of opaque material, the aperture area in the IPS mode liquid crystal display is reduced and the transmittance thereof is also degraded. As a result, so as to obtain a proper brightness, an intensified backlight is required which incurs too much power consumption.
So as to solve the foregoing problems, a method to form the electrodes 2a,2b with a transparent material has been proposed. In such a liquid crystal liquid display, the aperture ratio is often increased, however the transmittance is often not improved. That is to say, to produce an in-plane electric field, a distance l between the electrodes 2a,2b must be set to be greater than a distance between upper and lower substrates. To obtain a suitable intensity of the electric field, the electrodes 2a and 2b have relatively large dimension of width, for example, 10 to 20 xcexcm. 
However, if the electrodes have such a large dimension of width, the liquid crystal molecules positioned right above the upper surfaces of the electrodes 12 and 14 do not move thereby forming equipotential lines. As the result, since the liquid crystal molecules positioned right above the upper surfaces of the electrodes continue to hold an initial configuration even in the presence of the electric field, the transmittance is little increased.
The present invention is directed to provide a liquid crystal display having an improved picture quality.
Further, the present invention is directed to improve transmittance and aperture ratio of the liquid crystal display.
So as to accomplish foregoing objects of this invention, the liquid crystal display having high transmittance and high aperture ratio comprising:
an upper and a lower substrate opposed each other to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining sub-pixel regions;
a counter electrode formed on each sub-pixel region of the lower substrate;
a pixel electrode formed on each sub-pixel region of the lower substrate, wherein the counter electrode and the pixel electrode form an electric field;
a thin film transistor formed at an intersection of the gate bus line and the data bus line, and switching a signal transmitted from the data bus line into the pixel electrode when the gate bus line is selected; and
a homogeneous alignment layer formed on inner surfaces of the upper and lower substrates,
wherein a first electric field is formed between the counter and pixel electrodes of a selected sub-pixel among the sub-pixel regions, and the electric field is formed as a diagonal line with respect to the gate bus line and the data bus line,
wherein a second electric field is formed between the counter electrode and the pixel electrode of another sub-pixel adjacent to said selected sub-pixel, and the second electric field is formed as a diagonal line to make a symmetry with the first electric field.
The liquid crystal display further comprises:
an upper and a lower substrate opposed to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining sub-pixel regions;
a counter electrode including a body of a rectangular frame shape formed at each sub-pixel region and at least a diagonal branch dividing a region surrounded by the body;
a pixel electrode including a first electrode which is formed to be overlapped with a selected portion of the body of the counter electrode, and second electrodes being extended from the first electrode and disposed parallel to diagonal branches being formed at their corresponding sub-pixels;
a thin film transistor formed at an intersection of the gate bus line and the data bus line, and switching a signal transmitted from the data bus line into the pixel electrode when the gate bus line is selected; and
a homogeneous alignment layer formed on inner surfaces of the upper and lower substrates,
wherein the diagonal branches and the second electrodes at a selected sub-pixel region are extended as diagonal lines with respect to the gate bus line and the data bus line,
wherein other diagonal branches and second electrodes at other sub-pixel regions adjoining in all directions to said selected sub-pixel region are arranged to make a symmetry together with the diagonal branches and the second electrodes of the selected sub-pixel region,
wherein the counter and pixel electrodes are made of a transparent conductor, and widths of the diagonal branches of the counter electrode and the second electrodes of the pixel electrodes are properly set so that the electric field being generated between the counter and pixel electrodes may affect the electrodes entirely.
The liquid crystal display still comprises:
an upper and a lower substrate opposed to be spaced apart;
a liquid crystal layer interposed between inner surfaces of the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules;
a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining sub-pixel regions;
a counter electrode of a rectangular plate shape formed at a sub-pixel region;
a pixel electrode including a first electrode formed to be overlapped with a selected portion of the counter electrode, and a second electrode being extended from the first electrode in the form of a diagonal line;
a thin film transistor formed at an intersection of the gate bus line and the data bus line, and switching a signal transmitted from the data bus line into the pixel electrode when the gate bus line is selected; and
a homogeneous alignment layer formed on inner surfaces of the upper and lower substrates,
wherein the second electrode within a selected sub-pixel region extended as a diagonal line of the gate bus line and the data bus line,
wherein the second electrode within other sub-pixel regions adjoining in all directions to the selected sub-pixel region is arranged to make a symmetry with the selected sub-pixel region,
wherein the counter and pixel electrodes are made of a transparent conductor.